Bimetal



Oct. 19,1943; R. G. WALTENBERG 2,332,416

BIMETAL Filed Nov. 4, 1940 )7o/770152@ wife/26e@ ATTORN EYS Patented Oct. 1,9,`

BIMETAL Romaine G. Waltenberg, Roselle, N. J., assignor to The H. A. Wilson Company, Newark, N. J a corporation oi New Jersey Application November 4, 1940, Serial No. 364,124 2 Claims. (CL 297-.-15)

This invention is concerned with-thermally responsive bimetals employed for determination or control of temperature or temperature change in heating devices and the like. The invention provides improved'steel bimetals of high cor- 5 rosion resistance and great sensitivity so that they are capable of deilecting markedly and exerting great force per unit of weight with small temperature change.

Thermally-responsive bimetals, hereinafter reh, ierred tosimply as bimetals comprise two 01"'--r more metal members (sometimes called componen or sides having diiierent rates of expansion under the iniluence of heat and joined together in a unitary structure that changes l5 shape as it is heated or cooled. Thus, the bimetal may comprise two steel strips of different composition and expansibilities bonded together side by side. When the bimetal is heated, its two sides expand unequally so that it bends in an arc. 2o

Bimetals frequently are exposed to severe oxidizing conditions (for example, when immersed in water) and in such cases their life is short unless they offer high resistance to corrosion.

Heretofore customary bimetals in which both high-expanding and low-expanding sides arev steels are capable oi' withstanding relativelyhigh temperature without acquiring permanent set,

y but heretoforesuchvbimetals have not afforded both high ,corrosion resistance and high sensi- '30 tivity-at least to an adequate extent. In order to obtain high corrosion resistance in steel bi-l A metals, it has been necessary tosacrice sensitivity and. conversely, steel bimetals of high sensitivity have been relatively susceptible to cora5 rosion. e

Failure of bimetals through corrosion may come about because the individual sides have insumclent resistance to the oxidizing innuences to which they are .exposed or because of galvanlc 40 action between the sides, especially at the 'bond between them. Thus. metals which are resistant to corrosion if taken by themselves may fail to resist corrosion if taken together in the fbimetal.

As a result of my investigations, I have de- 45 veloped a steel bimetal having both great sensitivity and high corrosion resistance and which can be employed under conditions that are too severe for heretofore customary bimetals. The

components of the bimetal of my invention in- 150Y dividually have high resistance'to corrosion and are compatible with each other so that the effect of galvanic action is minimized and the bond between components is corrosion-resistant. More--` over, the rates oi the expansion o! the compo-- nents with temperature are so different that the bimetal is very sensitive, deflecting markedly with small changes in temperature and exerting great force per unit of weight. Both components of the bimetal of my invention are nickel steels, the low-expanding component being of the Invar type, i. e., a steel in which the sum of nickel and cobalt contents is very -high and the high-expanding side being an austenitic steel containing substantially less nickel than the low-ex.

panding side but with appreciable proportions of chromium and molybdenum. Thus, the low-expanding side may be a steel with a nickel content ranging from about to about 38% and a cobalt content ranging from about 2% to about 10%, the combined nickel and cobalt content being of theorder of The high-expanding side may be a steel containing chromium in proportions ranging from 7l2% plus sufficient nickel to render the steel austenitic (say 1'7-20%) and a substantial proportion of molybdenum (say of the order of 3%).

The bond between the components of my bimetal may be and preferably is autogenous, i. e., the two components are fastened together dlrctly withoutan intermediate layer. The bond may be formed in accordance with heretofore customary practice, for example, by Weldingtogether billets ofthe respective components disposed side by side and rolling out the resulting laminated mass to the desired thickness. Such a bond is strong and lasting, if, as in the case of my bimetal, the sides themselves and the bond are proof against oxidizing iniluences.

,Invar type steels are .well-,known and, as

indicated above, are characterized by a low rate of expansion with temperature. Invar type steels,

Considered generally, may hVe a Combined nickel and cobalt content (if cobalt is present) rang-A ing from 30-55 The cobalt content of the Invar type steel: component of the bimetal of my invention should lie in the range of about 2% to about 10% and the nickel content preferably should lie in the range of 30% to 38%. The combined nickel and cobalt content of the low-expanding component should beywell within the general Invar range of Sii-% and for best results should lie in the range of 36-42 The low expanding component may and, in iact, Ashould contain a substantial proportion of cl'n'omii'nrrI fw say 5% to 8%. The balance of -metal in the low,``

expanding component is essentially iron, i. e.,

i elemental ironwith small proportions of usual commercial ingredients. such ascarbon, phosphorus, sulfur. :silicon and manganese. It is, of

V ing austenitic structure at room temperature and,

course, desirable to keep these ingredients at a minimum and in particular the manganese c'ontent should not be permitted to exceed 1% and preferably should be below 95%.

The high-expanding side of my bimetal is an austenitic steel as distinguished from a steel of the Invar type, that is to say the alloy ingredients employed in the high-expanding side, such as nickel and chromium, are not present in propor- A tion greatly exceeding that required for preservl of course, at the temperature of service. Both chromium and nickel tend to preserve austenitic structure. A Straight nickel steel is austenitic in character with a nickel content of about The nickel content may be reduced substantially and still preserve austenitic structure of the steel if chromium is present. Thus, such steel containing 3% chromium is austenitic if it also contains 21-22% nickel. It the chromium content is raised to 9%, the nickel content may be reduced to about 18%. Consequently, a high-expanding component of the bimetal of my invention which contains 'l-12% chromium (the preferred proportion) requires nickel in proportion ranging from about 17% to about 20%. The high-expanding component. however, must also contain substantial proportions of molybdenum (say 2% to 4%). The balance oi' the metal present in the. low-expanding side is essentially iron and should be relatively free of ingredients auch as carbon, phosphorus and silicon. However, 4 manganese may be present in proportions of the order of 1%| although such a high manganese content is not essential.

The preferred range of ingredients in both the vlow-expanding side and the high-expanding side of the bimetal of my invention is indicated on the following table:

Table I 40 Lowexpanding component Highexpanding component Ingredients A speciilc example of a bimetal of my invention possessing both great sensitivity and long life, due to high corrosion resistance of the components both individually and collectively. 1s as follows: Table II Ingredients The properties of the bimetal of Table n (which is characteristic of all the bimetala of my invention) may best be' described with reference tothe accompanyingdrawing in which aensitivity ,is shown by two graphs. Thus. curve A is a tures.

` Linfluence brings about failure of the bond be per unit thickness. It is customary to consider ilexivity in the terms of a constant K which depends upon the diierences in thermal expansion, in ratio of elastic moduli and in thickness of the components (sides) of the bimetal, the relationship between F and K and other factors being expressed by the equation:

It will be seen that flexivity or K is a measure of the sensitivity of a bimetal to temperature change at a given temperature level. The values for the K of the bimetal of the Table II at different temperatures are plotted on the graph as curve B. Referring to this curvel it will be observed that the maximum exivity of the bimetal occurs for agtemperature range up to about 300 F. and the bimetal is particularly sensitive up to this temperature. At temperatures higher than 300 F., the ilexivity decreases.

As indicated above, curve A on the graph shows the deection of the bimetal at various tempera- It will be observed that deilection Iincreases rapidly and more or less constantly -to a temperature of about 450 F.500 F. At higher temperatures deflection increases at a substantially lower rate, so from the standpoint of deilection, the preferred operating range of the bimetal is below about 500 F.

To summarize, the temperature range of useful deflection of the bimetal of Table II is from about 50" C. to about +500 F. 'I'he temperature range of maximum sensitivity of the bime'tal, as indicated by curve B. is from +50 F. to about +300 F'. The K value of the bimetal over the temperature range of maximum sensitivity is about .0000053. 'Ihe bimetal has an electrical resistivity at '10 F. of about 400 ohms per sq. mil.ft., and its modulus of elasticity in pounds per square inch is about 25.000.000.

The bimetal of Table 1I (like the others of -this invention) is strongly resistant to corrosion and need not be protected with a coating or plating even when in use under severe oxidizing conditions. Its temperature deflection rate is sub stantially greater than that of bronze-steel bimetals now in use and exerts substantially greater force per unit of weight for changes in temperature.

The bimetal has a long life and does not fail by splitting, which occurs in other bimetals in which galvanic action with or without oxidizing tween the components. l

1. A steel bimetal adapted to change shape as its temperature is changed and comprising two steel components of diiferent expansibilities welded directly together, the individual components and the bond between them being strongly resistant to corrosion by galvanic action between the g components, the low-expanding component and the high-expanding component being respec- 2,ss2,41e 3 tively of the Invar and the austenltlc types and nents and the bond between them being strongly having the following analyses: v resistant to corrosion by galvanic action between the components, the low-expanding component 'Low-expanding component High-expanding component and the high-expanding CODIDODent being Ie- 5 speetlvely ot the Invar and the austenitic type Per cent by Ingredients pegvgtby and having the following analyses:

Ingredients weight V Low-expanding component High-expanding component z-m 72T; 1 17:1) 10 Per cent b Per cent b I f3 (i) Ingrediente weight y Ingredients weight y C 2-10 Iggcg'e proportions up to 8 per cent gg and NL H l.' 2. A steel bimetal adapted to change shape as l5 Fe Balance F its temperature is changed and comprising two steel components of different expansibillties ROMAINE welded directly together, the individual compo- 

